Karnal Bunt Lookalike Is Unmasked, Helping SE Wheat Growers

This year, southeastern wheat growers should face less risk of a quarantine
on their crop due to Karnal bunt disease. That's because
Agricultural Research Service scientists
discovered that light and scanning electron microscopy (SEM) can distinguish
Karnal bunt from a comparatively harmless lookalike fungus on ryegrass.

Sometimes, tiny amounts of ryegrass infected with the lookalike are
inadvertently harvested along with the wheat. Until now, available tests have
incorrectly identified this fungus as Karnal bunt. As a result, restrictions
were placed on the movement of wheat in 1996 and early 1997 from Alabama,
Georgia, Florida and Tennessee because it was suspected of being infected with
Karnal bunt.

But the ARS technique quickly showed each of the 70 wheat samples collected
from southeastern farms in 1996 was contaminated with the lookalike fungus, not
Karnal bunt. As a result, in March 1997, these restrictions were lifted from the
counties where the suspect samples originated. Federal plant quarantine
officials now use the ARS technique as a "first cut" to decide if
possible quarantine actions are needed. If the test results indicate a sample is
Karnal bunt, they go back and look for bunted wheat seeds.

ARS mycologist Lisa A. Castlebury in Beltsville, Md., developed the
technique. It uses light and SEMs to characterize teliospores (fungal seeds) of
dried and fresh specimens of both fungi.

After examining the two fungi's shape, size, surface characteristics and
color, Castlebury determined that these characteristics can be used to tell
these two fungi apart. She validated the technique by rigorously comparing her
test's results with known differences between the two fungi. Mature teliospores
of Karnal bunt (T. indica) on wheat are dark red-brown, often opaque.
Fine spines densely cover the outer seed coat. Teliospores of the unnamed Tilletia
species on ryegrass range from pale yellow or golden to dark brown; spines are
thicker and more widely spaced.

In 1996 and 1997, U.S. wheat export markets had a total average value of
about $5 billion a year. Most of this market was threatened by the ryegrass
fungus.